A typical clutch assembly incorporates a rotating clutch disk plate selectively secured against a facing plate. This facing plate is directly secured to the fly wheel. The clutch disk plate is connected via a spline to the main shaft, which extends in opposite directions and through both plates. One end of the main shaft goes through the clutch disk plate and into the transmission. The other end goes through the facing plate, the flywheel and on into the crankshaft of the engine. There is a support bearing near each of the plates to support the main shaft and accurately position the clutch disk plate. However, certain manufacturers of motorcycles have elected to place the clutch assemblies so that they are supported with only a single bearing on the inward side, providing only limited support to the entire clutch assembly and transmission input shaft or main shaft.
To illustrate a common set of problems found in a multitude of motorcycle clutch designs, we will refer to a “typical” motorcycle wet clutch assembly, which is composed of an adjusting plate, spring assembly, pressure plate, adjusting screw, disk assembly, inner hub, clutch shell, pilot bearing, and transmission input shaft or main shaft. The disk assembly consists of 6-8 fiber disks “stacked” in a parallel arrangement along a common axis between parallel metal plates. The disk assembly is connected to the inner hub via splines on the metal disks. The disk assembly is connected to the clutch shell via extended legs on the fiber disks. The clutch is released or disengaged by the push rod exerting force on the pressure plate, which is permitted to move outwardly, by compressing the spring assembly. The spring assembly being fixed outwardly by the adjusting plate, which is secured via bolts to the inner hub. The amount of compression being determined by the positioning of this adjustment plate relative to the bosses on the inner hub. Different positioning being determined by shims or an adjustment capability built into the adjustment plate itself. Disengagement permits the inner hub to move freely of the clutch shell and rotate freely on the pilot bearing. The inner hub is directly connected to the end of the transmission input shaft via a keyway and nut. The main shaft then proceeds inwardly through the pilot bearing in the clutch shell, then through an inward support bearing and subsequently into the interior of the transmission output shaft and subsequently into the transmission. The clutch shell is connected to the engine by a chain which attaches to the drive sprocket on the clutch shell. The disengagement of the rotation of the clutch shell from the inner hub is what achieves the disengagement of the motor from the transmission. However, the entire clutch assembly is outwardly positioned from the inward support bearing, its only means of support.
As previously noted, there are a multitude of motorcycle clutch designs, and a variety of terminology used in shop manuals and elsewhere to describe them. In our discussion, we shall rely on the fact that the outer clutch shell drives the inner clutch hub when the clutch is engaged. That is, the clutch shell does the driving and the inner hub gets driven. Consequently, the clutch disks, which interconnect with the clutch shell, will be referred to as the drive plates. These are typically made of fiber. The clutch disks, which interconnect with the inner clutch hub, will be referred to as the driven plates. These are typically made of steel. However, in some clutch designs the roles of the steel and fiber plates are interchanged. In our “typical” wet clutch design the drive plates are notched at their peripheries to receive the keyways formed in the clutch shell. In other designs, the periphery of the drive plate may be a spline, which interfaces with a mating spine in the clutch shell. Similarly there are various ways to mate the driven plates with the driven inner clutch hub. Also in our “typical” wet clutch design the spring assembly is fixed outwardly by the adjusting plate, which is secured via bolts to the inner hub. In other designs, there may be no adjusting plate. The spring assembly being restricted from an outward movement by a retaining ring, which is seated onto the spring and slips into a receiving slot in the inner hub thereby sandwiching the pressure plate between the clutch plates and spring, by utilizing the inner hub directly. In the “typical” wet clutch design the clutch shell is connected to the engine by a chain, which attaches to the drive sprocket on the clutch shell. In “dry” clutch designs, a belt often replaces this chain. In the “typical” wet clutch design the inner hub is directly connected to the end of the transmission input shaft via a keyway and nut. In other designs, this connection is implemented using a snap ring in lieu of a nut. In the “typical” wet clutch design, a pushing force actuated from the far or inward side of the pressure plate disengages the clutch. In other clutch designs the pushing force is actuated from the near or outward side of the pressure plate. In yet other designs, a pulling force actuated from the near side is used. Many of these alternative designs will be addressed subsequently.
Returning now to the use of our “typical” wet clutch design, we will illustrate a common set of problems found in motorcycle clutches. The energy from the engine is typically supplied by a chain drive which pulls the entire clutch assembly forward. This forward movement, of the entire clutch assembly, presses the transmission input shaft against one side of the inward support bearing, causing rapid wear. This forward movement of the clutch also causes misalignment of the clutch release elements and consequently the clutch is no longer able to be fully disengaged. Without proper disengagement of the clutch, shifting up or down cannot be done without grinding of gears, clanking, or other difficulties since the clutch is now binding or dragging when it should be completely disengaged. The forward movement of the clutch assembly also causes unnecessary transmission wear. The transmission input shaft passes through the transmission output shaft and on into the transmission. Any misalignment or binding of these two shafts causes premature wearing of these shafts and reduces the overall power output of the motorcycle. As the transmission input shaft enters into the transmission, its misalignment will also affect any gears, bushings, and bearings which it impacts upon. All of these problems, caused by the forward movement of the clutch assembly, are exacerbated as wear occurs and accumulates on the inward support bearing.
A further problem is looseness in the linkage between the rear (drive) wheel and the engine, commonly called “backlash.” A certain amount of backlash is necessary. However, it is desirable to minimize this looseness as much as possible. Fore and aft motion of the clutch assembly relative to the engine and the rear wheel provide a significant component of motorcycle drive train backlash. An unstabilized clutch assembly significantly contributes to the drive train backlash.
An additional problem in motorcycle clutches is starter drive inefficiency. The motorcycle starter drive engages the ring gear of the clutch shell of the clutch assembly which also is connected via the drive sprocket to the engine via chain. If the motorcycle is started while in neutral, the starter drive will force the clutch assembly away from the starter drive and cause a binding of the gears in the starter drive and ring gear. If the motorcycle is started while in gear, the engagement of the clutch shell by the starter drive, again urges the clutch to move away from the starter drive, causing the clutch to partially engage, increasing the energy needed to start the motorcycle engine.
Moreover, when disengaged, the motorcycle clutch permits the inner hub to spin from its own inertia, further delaying and/or interfering with smooth and efficient clutch and transmission operation.